Greenstone Belt. by Heikki Salmirinne and Pertti Turunen. Geological Survey of Finland, P.O. Box 77, FI-96101, Rovaniemi, Finland.

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1 Gold in the Central Lapland Greenstone Belt, Finland V. Juhani Ojala (ed.) Geological Survey of Finland, Special Paper 44, , Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt by Salmirinne, H. & Turunen, P Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt.Geological Survey of Finland, Special Paper 44, , 12 figures and 1 table. A short review with some examples of petrophysical and ground geophysical work done by the Geological Survey of Finland for gold exploration in the Central Lapland Greenstone Belt (CLGB) is presented. Although gold occurs in such low concentrations that it does not give any direct geophysical response, gold mineralized geological structures such as faults, shear zones, lithological units, alteration zones and associated minerals can usually be located by geophysical methods. The applicability of geophysical methods depends on the contrast in the physical properties of the target and the country rocks. According to regional petrophysical data the gold mineralized and altered greenstones differ slightly from the unaltered rocks in terms of density and magnetic susceptibility. Apparent resistivity, chargeability, and gamma radiation show that depending on the contrast in physical properties of the gold mineralized and the unaltered country rocks, ground geophysical methods can in many cases be used to locate and study gold mineralization indirectly. Electric or electromagnetic methods are an obvious choice in exploring for gold if sulphides are included. The most important geophysical methods used are magnetics together with electromagnetic VLF-R or HLEM. Magnetic anomalies provide information about geological units, faults, and shear and alteration zones. Electromagnetic anomalies are caused by graphite, sulphides and fractures containing water. IP may detect disseminated sulphides and SP is used to map and classify conductive sulphide and graphite occurrences. Since gold mineralization is commonly related with potassic alteration, radiometric methods may prove to be useful. Key words (Georef Thesaurus, AGI): gold ores, mineral exploration, Central Lapland Greenstone Belt, geophysical methods, ground methods, petrophysics, Paleoproterozoic, Kittilä, Sodankylä, Lapland Province, Finland. Geological Survey of Finland, P.O. Box 77, FI-96101, Rovaniemi, Finland. heikki.salmirinne@gtk.fi, pertti.turunen@gtk.fi 209

2 Introduction The physical properties of gold (Au), density kg/m 3 and electrical conductivity 5*10 7 S/m, are one of the most anomalous of all elements. In spite of this it is almost impossible to get direct geophysical response from gold, because of its low grade in deposits (Doyle, 1990). Nevertheless, geological structures such as faults and shear zones, lithological units, alteration zones and minerals (e.g. pyrite), that are associated with gold, can often be mapped by geophysical methods (Paterson and Hallof, 1991). Known characteristics of orogenic gold deposits in greenstone belts are (e.g. Airo, 2002; Allibone et al., 2002; Groves and Foster, 1991; Paterson and Hallof, 1991) commonly sited adjacent to crustal-scale shear zones on district scale sited in affiliated smaller faults or shear zones geometrically related to crustal-scale shear zones host rock can be almost any rock type detectable geophysical anomalies can be caused by the alteration haloes (common alteration processes includes carbonatization, sericitisation and silicification) Crustal-scale structures, faults and shear zones commonly produce observable geophysical anomalies of which aeromagnetic and gravity surveys are the most useful methods. Detailed ground geophysical surveys are used to detect smaller-scale faults and shear zones, alteration zones and minerals associated with gold to guide drilling. Although the geophysical methods form an essential part of gold exploration, particularly where outcrops are rare, one has to remember that the usefulness of the various methods depends on the contrast between the physical properties of the target and the host rock. Because orogenic gold mineralization can occur in various rocks type, there exist a wide variety of geophysical anomalies produced by the occurrences of different types. This means that the explorer must have experience, persistence and luck; Gold is where you find it. Petrophysical characteristics of rocks Petrophysical data can be acquired by two means. The more accurate and repeatable method is to use laboratory equipment to measure the rock samples. The other way is to lower a probe into a drill hole and measure the properties, not so accurately as in the laboratory, but in-situ. These two methods do not replace each other as some of the physical properties can be measured only in the laboratory conditions (say, remanent magnetization), and some only in the drill holes (say, temperature). Some of the most commonly used properties, viz. density, magnetic susceptibility, and gamma radiation intensity, can be recorded in a laboratory or drillhole whereas apparent resistivity and chargeability belong to the drill hole logging category in practical mineral exploration. The electrical properties could be determined from drill core samples, but a denser spatial coverage together with practically real-time availability and lower cost make loggings more attractive. In the national petrophysical database maintained by the Geological survey of Finland (GTK) (Korhonen et al., 1993; Säävuori and Hänninen, 1997) there are about 6500 rock samples gathered from the area of the CLGB. A summary of petrophysical properties (medians of bulk density, magnetic susceptibility and intensity of remanence) of different rocks types, measured in the laboratory, is presented in Table samples are from mafic volcanic rocks (greenstones). Density of mafic volcanic rocks is higher than other rocktypes. The density of granites is about 320 kg/m 3, and quartzites about 290 kg/m 3, lower than the average density of greenstones. Median of susceptibility presented in table 1, does not respect typical bimodal susceptibility distribution of Precambrian rocks (Puranen, 1989) into para- and ferromagnetic subsets. Susceptibility histograms should be used to identify these parts and median can be used just for a rough comparison between different rocktypes. The susceptibility of mafic volcanic rocks shows some variation, but is generally lower than in other rocks. On aeromagnetic maps, greenstones produce relatively low magnetic field values. Generally low magnetization is caused by intense regional hydrothermal alteration, predating regional metamorphism (Lehtonen et al., 1998). Locally, especially in marginal areas, where highly magnetic layers have been tectonically repeated and abundance of magnetite was increased in certain layers during regional metamorphism, strong magnetic varieties are more common (Lehtonen et al., 1998). There are no laboratory conductivity measurements in the database, but airborne electromagnetic data show numerous conductitivity anomalies within volcanic-metasedimentary rocks, mainly caused by graphite-bearing schists. 210

3 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt Table 1. Petrophysical properties of rocks in Central Lapland Greenstone Belt. Data from GTK s national petrophysical database. Rocktype Density [kg/m3] Susceptibility [10-6 SI] Remanence [10-3 A/m] Count Median stddev Count Median Count Median Quartzite Mica schist Skarn Dolomite Mylonite Phyllite Carbonate rock Black schist Sericite schist Conglomerate Graywacke Granite Monzonite Gneiss Granodiorite Diorite Quartz diorite Gabbro Granulite Quartz-feldspar schist Pegmatite Quartz / chert Quartz porphyry Porphyry Feldspar porphyry Quartz dike Diabase Serpentinite Acidic/felsic volcanic rock Intermediate volcanic rock Mafic volcanic rock Ultramafic volcanic rock In the CLGB prospects petrophysical drill hole loggings have also been done. Apparent resistivity, chargeability, and gamma radiation are typical logging parameters. Two other parameters, density and susceptibility, can be measured with better accuracy in the laboratory, but the representativeness of the two modes are in the same class. The bulk density, measured in the laboratory, is a combination of dry and saturated density depending on the saturation degree of the sample. This vagueness causes only tiny errors in density due to small porosity of crystalline rocks. In hole logging gives in-situ information on the physical properties, i.e. the real properties that produce the anomalies mapped in the field. The gold content in rocks is, excluding exceptional cases, so low that it has neither effect on the rock s physical properties nor causes geophysical anomalies. Instead of gold, one has to explore and study such rock 211

4 units that are somehow connected to gold, in other words, to find the gold indirectly. In the three cases that are described below, gold is related to certain altered rock units or sulphide mineralizations, that can be detected by geophysical methods. Kaaresselkä. The ground water table at Kaaresselkä is typically 50 meter below the ground surface. The overburden and bedrock above this level are poor electrical conductors, which causes the logging responses to be subdued, especially the galvanic methods. Figure 1 shows histograms of gamma-gamma density, magnetic susceptibility, Wenner apparent resistivity, chargeability, and total gamma radiation from 24 drill hole sections below the ground water table. The rocks have been classified into gold-bearing mylonites and other rocks. In the apparent resistivity and gamma radiation, there are satisfactory differences between the two classes, but in the density, susceptibility and chargeability, the distributions of the parameter values overlap too much to have use for the practical field exploration. In the case of susceptibility, the peak values are different, but the overlap of the two distributions is substantial. Electrical and electromagnetic methods are best suited to gold exploration in the Kaaresselkä area. The use on gamma radiation is insignificant in the field mapping as the radiation attenuates to zero within 30 cm of the source. However, in drill hole logging, the gamma radiation can be used to detect the potassic alteration zone that is commonly related to the gold mineralization. Loukinen. In the Loukinen occurrence the gold mineralization is hosted by breccias containing graphite. Rocks that contain graphite are good electrical conductors and are occasionally radioactive, because geochemical properties of uranium make it susceptible to deposit in sapropels. Figure 2 shows the physical properties logged from a borehole, together with gold content. The correlation between gamma radiation and gold content is very good. Further, apparent resistivity and chargeability correlate almost equally well with gold. The apparent resistivity minima are accompanied by chargeability maxima and vice versa. This means that both are caused by electrical conductors, and the polarization due to disseminated sulphides is not remarkable. The electrical anomalies can be explained by the black schist. Susceptibility is low and correlates weakly but positively with gold whereas the correlation between density and gold content is weakly negative. In the Loukinen area petrophysical data suggests that the induced polarization may be effective as a ground survey method, and gamma radiation in the logging environment. The histograms in Figure 3 are based on 15 drill hole sections and show that the difference in chargeability between gold-bearing and other rocks is significant, more that one decade. Another clear difference is in the gamma radiation histograms, and the variation is even larger than shown in Figure 3, because the highest gamma peaks would go up to more than one thousand. There is, however, considerable overlap. In the three other parameters, the distributions of the gold-bearing and other rocks overlap. Below it will be shown that even if the logged chargeability seems to be a very good parameter, in practice black schists make its use difficult. The incompatibility between the histograms of apparent resistivity and chargeability in Figure 3 is interesting. It is not clear how the gold mineralization is related to black schists and sulphides, which can be detected with electrical methods regardless of gold content. The chargeability works in much the same way, but is affected by polarization effects. For some reason, possibly mineralogical or structural, mineralized rocks polarize more than barren rocks. This phenomenon is especially clear in Figure 4, where chargeability, gamma radiation, and gold content from the drill hole R511 are shown. The data consist of samples of graphite-bearing breccias. Both the gamma radiation and gold content correlate well with chargeability. This correlation is detectable in most drill holes near R511, but its extrapolation outward should be done with care. Iso-Kuotko. In the Iso-Kuotko area any rocktype can be mineralized and geophysical exploration is based on detecting sulphides in alteration zones with electric or preferably with electromagnetic methods. Figure 5 shows the correlation between gold and sulfur and the associated variations in the apparent resistivity and chargeability. The correlation between gold and sulfur is clear and pronounced, which suggests that the gold mineralization is related with sulphides. Most sulphides are good conductors and polarize easily, and make finding gold possible in an indirect way. The size of the circles in the Figure 5 represents the apparent resistivities or chargeabilities as described in the legend. The lowest apparent resistivities are located in the right upper corner where sulfur and gold contents are highest. With chargeability, the situation is more complex as the largest circles are distributed evenly to cover the sulfur and gold variation range. This means that the apparent resistivity can be used to classify the sulphur, and that way gold content, but the chargeability can not. On the other hand, chargeability is more efficient in locating zones of weak gold content than apparent resistivity. The logging of chargeability together with apparent resistivity seems to be justified. 212

5 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt Fig. 1. Histograms of the physical properties of rocks from Kaaresselkä. 213

6 Fig. 2. Petrophysical logs and gold analyses from the drill hole R510 in Loukinen. 214

7 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt Fig. 3. Histograms of the physical properties of rocks from Loukinen. 215

8 The Iso-Kuotko area is not an ideal place for the drill hole surveys as the ground water level is very deep, almost 100 meter below the surface. Deep water table makes electric loggings difficult because there is no electrolyte present in the topmost parts of the bedrock. During surveys, water was poured into holes to make galvanic measurements somehow possible, but in spite of this the noise level at depths above the water table was rather high. This causes some noise in the logging data in Figure 5. Fig. 4. Interdependence of chargeability, gamma radiation and gold content in the drill hole R511 in Loukinen. Fig. 5. Interdependence of sulfur content, gold content, conductivity and chargeability in Iso-Kuotko. Ground methods used In the Central Lapland Greenstone Belt proterozoic volcanic and metasedimentary rocks cover an area of about km 2. In addition to high-quality systematic airborne surveys, detailed ground geophysical surveys have been completed, for both geological mapping and exploration purposes (Fig. 6). The magnetic method, in combination with electromagnetic VLF-R or HLEM (Slingram) surveys, have been the main methods used. Other methods include gravity and Induced Polarization (IP). For these surveys line spacing has been mostly 200, 100 or 50 m and station spacing 20 or 10 m. There are also some very detailed Self Potential (SP) and Mise-à-la-Masse surveys, which are not presented in Figure 6. Below, known gold deposits are discussed in the context of regional gravity data. Examples of detailed surveys used to explore specific targets are then presented. Regional gravity. In the early 1970 s, GTK in cooperation with the Finnish Geodetic Institute began regional gravimetric surveys in which station density has varied from 1 to 6 per km 2. In CLGB area, gravimetric measurements include points in an area of about km 2, with an average point spacing of 1 point/km 2. Finnish Geodetic Institute has measured 1252 points in the same area (Kääriäinen and Mäkinen, 1997). Figure 7 has been compiled from these data sets. Volcanic rocks within CLGB are associated with higher Bouguer anomaly values compared to the surrounding granitoids. In the eastern part of the area, mafic layered intrusions, Keivitsa and Koitelainen, give 216

9 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt Fig. 6. Ground geophysical index map of GTK s surveys in the Central Lapland Greenstone Belt plotted on a gray scale aeromagnetic map. Gold deposits and volcanic-sedimentary rocks of the Central Lapland Greenstone Belt are shown. Aeromagnetic data by GTK. Fig. 7. Regional Bouguer anomaly map of CLGB. Station density 1 point/km 2. Sirkka Line and known gold deposits are shown as dotted line and triangles, respectively. Lälleävuoma, Loukinen, Suurikuusikko, Iso-Kuotko and Kaaresselkä deposits have been labelled separately. 217

10 rise to gravity highs, as does the Lapland granulite belt in the NE-part of the area. Earlier geophysical interpretations (Elo et al., 1989, Lehtonen et al, 1998) suggest that the volcanic rocks dominated greenstone belt has a thickness of up to 6 km. The horizontal gradient of Bouguer anomaly indicates steep tectonic contacts between the greenstone belt and the surrounding granitoids. Most of the known gold deposits are situated along a tectonic contact zone, called the Sirkka Line, between volcanic and sedimentary rocks. However, the largest known deposit, Suurikuusikko, is situated along a N-S trending shear zone in the middle of Mg- and Fe-tholeiitic metavolcanic rocks. Weak negative Bouguer anomalies indicating shear zones and faults has been observed on gravity profiles measured across the zone (Lehtonen et al., 1998). These anomalies can be signals of intensely altered rocks within faults and shear zones such as in Suurikuusikko, and are worth of exploration in the future. Magnetic, EM VLF-R and HLEM. The magnetic method is widely used geophysical tool in the gold exploration (Doyle, 1990) and in CLGB also. Magnetic data provide information on geological units, faults and shear structures. In some cases, alteration zones can be observed, because ferromagnetic minerals in mafic volcanic rocks are destroyed by alteration processes resulting in magnetic lows. Magnetic data from the Lälleävuoma deposit is presented in Figure 8. The deposit is located within a sequence of basaltic and komatiitic metavolcanic rocks and fine-grained metasedimentary rocks. The Sirkka Line occurs nearby Fig. 8. Example of ground magnetic data from Lälleävuoma area. Total magnetic field was measured using proton precession magnetometer. Shaded relief map, illuminated from northeast. Line spacing 50 m and station spacing 10 m. Lälleävuoma deposit is marked by a yellow triangle. 218

11 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt (Eilu, 1999). Magnetite in talc-chlorite schists result in high magnetic intensity, whereas moderately carbonatizated rocks are seen as weak magnetic minima. VLF-R is the often measured together with magnetic surveys in CLGB area by GTK. It is the most favourite electromagnetic method in northern Finland because it is cheap, fieldwork is fast and cultural noise levels are low in sparsely populated areas. In the Lälleävuoma area, Geonics EM16R instrument was used to conduct VLF-R surveys. VLF-R transmitter DHO-38 (frequency 23.4 khz) in Germany was used. In the Lälleävuoma area, N-S trending metakomatiitic rocks stand out as a high resistive zone in the middle of the area (Fig. 9). The contacts between metakomatiites and conductive graphitic phyllites can be signified quite accurately. An example of the ground magnetic data with electromagnetic HLEM (slingram) data is presented in Figure 10 from the Suurikuusikko deposit. The deposit is associated with a 15 km long, N-S trending shear zone chiefly located in the contact zone between major Mg- and Fe-tholeiitic metavolcanic units. The area around the deposit is dominated by mafic pyroclastic metavolcanic rocks (Härkönen, 1997). The gold deposit consists of lens-shaped, sulphide-bearing bodies striking in the main shear zone direction. Gold is associated with arsenopyrite, pyrite and gersdorffite. The sulphide-bearing tuffites or phyllites and graphitic host rocks give the major response in both magnetic and electromagnetic methods. The Suurikuusikko shear zone is indicated as a negative anomaly in magnetic maps and, due to abundant graphite vein network and pyrrhotite, as a low in the resistivity maps. Magnetite have been essentially destructed all along the shear Fig. 9. Example of VLF-R phase data from Lälleävuoma area. Conductors can be seen as areas where phase angle is over 50 degrees. Line spacing 50 m, station spacing 10 m. Lälleävuoma deposit is marked by a yellow triangle. 219

12 Fig. 10. A) Magnetic residual anomaly map and B) a rough classification of magnetic and HLEM (slingram) anomalies along the Suurikuusikko shear zone. Gold occurrences (Riddarhyttan Resources AB, 2002) marked by red dashed line. Data combined from GTK s and Riddarhyttan Resources AB s datasets. 220

13 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt zone as the result of alteration. Magnetic anomalies are caused mainly by pyrrhotite that in some places is massive (Härkönen, 1997; Eilu, 1999). Figure 10 presents the magnetic residual anomaly and a rough classification of the magnetic and HLEM anomalies measured along the Suurikuusikko shear zone. Induced Polarization (IP). IP surveys have commonly been applied in gold exploration to locate sulphides in situations where sulphide concentration is low or moderate. Disseminated sulphide deposits in volcanic-sedimentary components of greenstone belts may not be detectable using electromagnetic methods (like VLF-R), but in some cases can be mapped using IP. In many areas of the CLGB, there are very conductive graphite-bearing schists that cause chargeability and conductivity anomalies and disturb and mask all other information. Time-domain measurements with a dipole-dipole array are commonly used, because this array is very sensitive to horizontal changes, and the effects of small local sources are larger than with other configurations. An example of an IP survey from Iso-Kuotko deposit is presented in Figure 11. Scintrex IPR-10 receiver and dipole-dipole electrode array (a=20m, n=2) with 50 m line spacing and 20 m station spacing was used. Iso-Kuotko deposit is related to the intersection of the NW-trending Kuotko Main Shear (KMZ) and the NE-trending extension of the Suurikuusikko Shear Zone (SSZ). The mineralization and the host rocks are within a sequence of mafic metavolcanic rocks and volcanogenic metasedimentary rocks (Eilu, 1999). The Kati lode is directly related to the KMZ and strike of the Tiira lode corresponds to the SSZ. The structural geology of the area is complicated and rocks are strongly altered by albitization, carbonatization and silicification. Soil cover is thin, only up to 5 m (Härkönen et al., 2000). In the Iso-Kuotko deposit, the Kati and Tiira lodes are indicated by VLF-R, HLEM and IP methods, as conductivity or chargeability anomalies. Sulphides (pyrite, arsenopyrite and pyrrhotite) are both semimassive and disseminated, and there are no graphite. Conductive structures in the Tiira lode are subvertical and the chargeability response of IP measurements is sharp and distinctive, whilst in the Kati lode goldbearing NW-SE trending horsetail-like thin quartzcarbonate veins with associated sulphides are dipping degrees to NE and, consequently, the anomaly shape is more wide and fuzzy (Fig. 11). Anomalies indicate also several overlapping conducting layers near the surface to north and west of the Kati lode (Härkönen et al., 2000). Self Potential (SP). In the areas where the water table is below the bedrock surface, the conditions are favourable for self potential differences to form. Anomalous potentials (hundreds of millivolts) can be observed over rocks bearing sulphide and graphite, or other conductive minerals. In many cases, large bog areas in northern Finland disturb measurements by masking the desirable response. However, the SP method has been infrequently, but successfully, used for mapping and classifying conductive sulphidic Fig. 11. IP chargeability anomaly map from Iso-Kuotko gold target. Gold-bearing sulphide lodes Kati and Tiira with drill hole locations are plotted. Trends of the Suurikuusikko and Kuotko shear zones are also shown. 221

14 Fig. 12. SP results from the Loukinen gold occurence. Line spacing 10 m and station spacing 5 m was used. In the western part of SP-area, bogs masks and the response was not as good. and graphitic formations. Data has been collected in the traditional manner by measuring the absolute potentials between nonpolarizing electrodes. One electrode is kept at a reference point while the other is moving. Commercial voltmeter with a sufficient high input impedance was used. The survey grid has usually been very dense so the resolution is good. An example from the Loukinen deposit is presented in Figure 12. Study area is located along the contact zone of ultramafic volcanic rocks and graphitic phyllites controlled by the Sirkka Line. Anomalies are caused by graphitic phyllites and partly weathered sulphides (pyrite, chalcopyrite, gersdorffite). DISCUSSION Geophysical methods used in the gold exploration in the CLGB have been applied quite widely. Ground magnetics in combination of EM VLF-R or HLEM have been the main methods used as the first step. Interpretation has been done mainly qualitatively, and the most interesting anomalies, assumed to be relevant for the appropriate problem, have been chosen for sampling and drilling. Magnetic anomalies provide information about geological units, faults, and shear and alteration zones. Electromagnetic anomalies are caused by graphite, sulphides and fractures containing water. As the second step other methods such as IP and SP have been used if considered to be useful. IP may detect disseminated sulphides and SP is used to map and classify conductive sulphide and graphite occurrences. The value of the methods used depends on the petrophysical contrasts of the local rocks. Therefore petrophysical data is very useful to detect differences between gold mineralized units and barren country rocks. Examples of petrophysical drill hole loggings presented above indicate that in favourable cases it is possible to detect gold critical units indirectly. However, the selection of the methods has been done based on the geological and geophysical reasons as well as costs and equipment availability. For these reasons e.g. modern reflection seismic method has not been used for small-scale gold exploration in CLGB, although in many cases it could considered to be useful to map gold critical structures. 222

15 Ground Geophysical Characteristics of Gold Targets in the Central Lapland Greenstone Belt REFERENCES Airo, M.-L., Aeromagnetic and aeroradiometric response to hydrothermal alteration. Surveys in Geophysics 23, Allibone, A.H., McCuaig, T. C., Harris, D., Etheridge, M., Munroe, S., Byrne, D., Amanor, J. & Gyapong, W Structural Controls on Gold Mineralization at the Ashanti Deposit, Obuasi, Ghana. Society of Economic Geologist, Special Publication 9, Doyle, H. A Geophysical exploration for gold A review. Geophysics, vol 55, no 2, Eilu, P FINNGOLD a public database on gold deposits in Finland. Geological Survey of Finland, Report of Investigation p, 1 figure, 1 table and 2 appendices. Elo, S., Lanne, E., Ruotoistenmäki, T. & Sindre, A Interpretation of gravity anomalies along the POLAR Profile in the northern Baltic Shield. Tectonophysics 162 (1 2), Groves, D. I and Foster, R. P Archean lode gold deposits. In: Foster, R. P. (ed.) Gold metallogeny and exploration. Blackie: Härkönen, I., Pankka, H.& Rossi, S The Iso-Kuotko gold prospects, northern Finland. Sumary report, C/ M06/2744/00/1/ p. + 9 appendixes. Härkönen, I Tutkimustyöselostus Kittilän kunnassa valtausalueilla Suurikuusikko 2 ja Rouravaara 1 10 (kaivosrekisterinumerot 5965/1, 6160/1, 6288/1 6288/9) suoritetuista kultatutkimuksista vuosina English summary: Gold exploration during within the exploration claims Suurikuusikko 2 and Rouravaara 1 10 (Mine reg. nos. 5965/1, 6160/1, 6288/1 6288/9). Geological Survey of Finland, unpublished report M 06/2743/97/1. 47 p. (in Finnish) Korhonen, J.V., Säävuori, H., Wennerström, M., Kivekäs, L. & Lähde, S One hundred seventy eight thousand petrophysical parameter determinations from the regional petrophysical programme. Geological Survey of Finland, Current research , Kääriäinen, J. & Mäkinen, J The gravity survey and results of the gravity survey of Finland Publications of the Finnish Geodetic Institute. N:o p. + 1 app. map. Lehtonen, M., Airo, M.-L., Eilu, P., Hanski, E., Kortelainen, V., Lanne, E., Manninen, T., Rastas, P., Räsänen, J. & Virransalo, P Kittilän vihreäkivialueen geologia: Lapin vulkaniittiprojektin raportti. Summary: The stratigraphy, petrology and geochemistry of the Kittilä greenstone area, northern Finland: a report of the Lapland Volcanite Project. Geological Survey of Finland, Report of Investigation p. + 1 app. map. Paterson, N.R. and Hallof, P.G Geophysical exploration for gold. In: Foster, R. P. (ed.) Gold metallogeny and exploration. Blackie: Puranen, R Susceptibilities, iron and magnetite content of Precambrian rocks in Finland. Geological Survey of Finland, Report of Investigation p. + 6 app. pages. Riddarhyttan Resources AB Länsprofiler av de mineraliserade zonerna i Suurikuusikko. Pressmeddelande Press release , (in Swedish) Säävuori, H. & Hänninen R Finnish petrophysical database. In: Korhonen, J. V. (ed.) Petrophysics in potential field interpretation: First Workshop for the Finnish Geophysical Crustal Model Program, August 1997, Espoo, Finland: abstracts. Espoo: Geological Survey of Finland,